The present invention relates to a liquid crystal display device, and in particular, to a technique useful for a driver-circuit-integrated liquid crystal display device having a display section and a drive circuit therefor fabricated on the same substrate.
Recently liquid crystal display devices have been widely used in equipment ranging from small display devices to display terminals for office automation equipment and the like. Basically, a liquid crystal display device includes a liquid crystal panel (also called a liquid display element or a liquid crystal cell) which has a layer of liquid crystal composition (a liquid crystal layer) sandwiched between a pair of insulating substrates, at least one of which is made of a transparent substrate (for example, a glass plate or a plastic substrate).
This liquid crystal panel produces an image by selectively applying voltages to various pixel-forming electrodes, and thereby changing orientation of liquid crystal molecules of the liquid crystal composition in desired pixels. Among the liquid crystal panels, a type is known in which pixels are arranged in a matrix configuration to form a display section. Liquid crystal panels having pixels arranged in a matrix can be roughly classified into two types, a simple matrix type and an active matrix type. The simple matrix type forms pixels at intersections of two strip electrodes intersecting each other which are disposed respectively on a pair of insulating substrates. On the other hand, the active matrix type has pixel electrodes and active elements (for example, thin film transistors) for pixel selection in respective pixels. By selecting desired ones from among the active elements, the active matrix type forms an image by pixel electrodes coupled to the selected active elements and a reference electrode facing the selected pixel electrodes.
The active matrix type liquid crystal display device are widely used as display devices for notebook personal computers and the like. In general, the active matrix type liquid crystal display devices employ a so-called vertical electric field type in which an electric field is applied between two electrodes disposed on the two substrates, respectively, so as to change orientation of liquid crystal molecules of the liquid crystal layer sandwiched between the two electrodes. Also, the liquid crystal display devices of the so-called horizontal electric field type (also called IPS (In-Plane Switching) type) have been put to practical use. The so-called horizontal electric field type applies electric fields in the liquid crystal layer approximately in parallel with major surfaces of the two substrates.
On the other hand, liquid crystal projectors have been put to practical use which employ liquid crystal display devices. The liquid crystal projectors irradiate illuminating light from a light source onto their liquid crystal panels and project images on the liquid crystal panels onto a screen. The liquid crystal projectors-employ two types of liquid crystal panels, the reflective and transmissive types. In the reflective type the liquid crystal panel, by making pixel electrodes light-reflective, and disposing structures such as wiring lines below the pixel electrodes, the approximately entire area of the display section can be used as a usable reflective surface, and therefore the reflective type is more advantageous than the transmissive type for realization of small-sized, high-definition and high-luminance liquid crystal panels.
In addition, the driver-circuit-integrated type liquid crystal display device is used as the active-matrix type liquid crystal display device for the liquid crystal projector, because the driver-circuit-integrated type liquid crystal display device has a driver circuit for driving pixel electrodes disposed also on a substrate on which the pixel electrodes are, and is capable of realizing a small-sized and high-definition liquid crystal display device.
Furthermore, among the driver-circuit-integrated type liquid crystal display devices, a reflective type liquid crystal display device (hereinafter sometimes called a Liquid Crystal on Silicon or an LCOS) is known in which has pixel electrodes and a driver circuit formed on a semiconductor substrate, but not on an insulating substrate.
In the driver-circuit-integrated type liquid crystal display devices, in a case where a D/A conversion (hereinafter sometimes called a digital-analog conversion) is employed which selects a gray scale voltage level to be supplied to a pixel electrode based upon display data in digital form, a problem arises in that, as the number of gray scale levels to be displayed is increased, the number of bits of display data is increased, and consequently, the size of circuit structures is excessively increased.
However, there is a tendency for output signals from video equipment to be provided in digital form, instead of analog form, and therefore, in the driver-circuit-integrated type liquid crystal display devices also, a driving method is desired in which the liquid crystal display device receives digital signals, and converts the digital signals into video signal voltages exhibiting plural gray scale voltage by using a drive circuit fabricated on the liquid crystal display panel.
As a method of producing a plural-gray-scale display in the driver-circuit-integrated type liquid crystal display device supplied with digital signal inputs, Japanese Patent Application Laid-Open No. 2,000-194,330 discloses a D/A conversion method of performing a D/A conversion by using a selector circuit configured to select a desired voltage level from a voltage varying in a staircase fashion.